The outside of left and right edge beams of the front part of a semi-trailer are usually equipped with a lifting leg to support the weight of the front part of the semi-trailer, to adjust the height of the front part of the semi-trailer and to adapt the inserting or withdrawing of the fifth wheel. When the semi-trailer is pulled to travel, inner legs retract to leave the ground so as to keep a safe distance.
A semi-trailer landing gear (leg) includes: outer legs mounted at the left and right sides of the front part of the semi-trailer, inner legs which extend and retract upwards and downwards in the outer legs, a screw and nut assembly which connects the inner and outer legs and transfers load, a two-speed gear transmission, a crank and a central shaft for connecting the left and right legs. In order to ensure the quality of the leg, except for optimizing the manufacturing process of the product to ensure the concentricity between the centerline of the outer legs and the center hole of the bearing cushion, between the centerline of the inner legs and the center hole of the loading plate, it is also important to optimize the design of the screw-nut which transfer load.
Traditional designs for a screw nut adopt a spherical nut, a two-side semicircle bossed nut or a hanging groove nut, etc. These kinds of nuts have their own features and all of them can fine-tune the concentricity between the axes of the screw and the nut. For example, the present applicant used a similar nut in the Chinese patent application No. 99236524.4 (with the title of “A Semi-trailer Landing Gear”) and obtained a Chinese utility model patent No. 200520064422.1 with the title of “A Screw-Nut for a Semi-trailer Landing Gear” for a similar nut. However, all of these nuts are single-layered. When the screw rotates, only angles of a single layer of the nut contact the inner wall of a quadrate pipe and the other portion of the nut hangs, and thus the nut under the rotating torque cannot be kept stable. Thus, the lifting capability of the leg is reduced, effectively meshed threads of the screw and the nut reduced and the moment and the shear of the threads increased. According to American standards AAR, experiments show that vibration can easily occur and loud noise is generated (80-85 db); the threads can easily deform, even the screw and the nut are stuck, causing failure of the experiment (the failure chance is 20%-25% or even higher). Years of practice proves that the threads of traditional nuts are easy to deform in application, or even the screw and the nut are stuck. As a result, rotation becomes difficult and service time of the screw and the nut is affected.